1. Field of the Invention
This invention relates to magneto-optic devices for sensing electric current and more particularly to such devices for measuring d-c currents which are compansated for drift attributable to variations in the properties of the optical components.
2. Prior Art
Magneto-optic current transformers have found widespread use as sensors for measuring large electric currents. In such devices, polarized light is passed through a sensor made of magneto-optic material where the plane of polarization is rotated through a small angle in response to the current being measured in accordance with a phenomenon known as the Faraday effect. Preferably, the sensor encircles the conductor carrying the current so that relative positioning of the sensor and conductor is not critical. The light beam is analyzed after is has passed through the sensor to determine the degree of rotation of the plane of polarization and therefore, the magnitude of the current.
In many current sensors, an analyzer splits the emerging polarized light into two orthogonal components, one rotated +45 degrees and other -45 degrees from the plane of polarization of the injected light. These two light components are converted by photodetectors into electric signals which are amplified in separate channels to produce signals which are typically labeled "a" and "b". The amount of rotation of the plane of polarization of the light beam is determined in an electronic module which electrically generates an output (a-b)/(a+b) which is proportional to the magnitude of the current. This output is independent of changes in the intensity of the light source and changes in attenuation of the optical components up to the analyzer. However, any changes in the relative effective gains of the parallel signal channels through the analyzer to the electronic module can produce a sizable error in the measured current. While it would be possible to periodically turn off the current and adjust the gain of one of the signal channels so that the output of the electronic module is zero, such a procedure is not practical in most applications of these sensors, such as in monitoring the current in a high voltage transmission system.
In order to provide drift free measurement of direct current signals, it is necessary to generate sufficient information to determine four quantities representative of the two orthogonal components of the polarized source and of the rotation due to the current. In our copending application Ser. No. 717,989 filed on Mar. 29, 1985, referred to above, the necessary information is generated by alternatively passing light of two different wavelengths through the sensor, and processing in separate channels electrical signals generated from the two orthogonal components of each of the emerging light beams. Since the Verdet constant of the magneto-optic sensor is a function of wavelength, the gain of the two processing channels is adjusted to maintain E.sub.1 V.sub.2 =E.sub.2 V.sub.1 where V.sub.1 and V.sub.2 are the Verdet constants and E.sub.1 and E.sub.2 are the quantities (a-b)/(a+b) at the two wavelengths respectively.
U.S. Pat. No. 4,243,936 discloses a magneto-optic current transformer in which an electrically controlled modulator alternately polarizes, in two orthogonal directions, light passed through the magneto-optic element. A photodetector in a single output channel converts the emerging alternately polarized light beam into an electrical signal which is demodulated to generate signals representative of the rotation of the alternately polarized light beam due to the current. Such a system, however, does not provide sufficient information to eliminate the effects of drift attributable to differences in the properties of the modulator in the two planes of polarization. This system also requires that the modulator, with its electrical interface, be located in close proximity to the magneto-optic sensor which is a disadvantage in a practical application of such a detector.
U.S. Pat. No. 3,743,929 discloses a magneto-optical device for measuring the current in a transmission line in which two light beams are passed in opposite directions through an optical path which includes a magneto-optical element between two polarizers which have their axes of polarization set at an angle to one another. Light from a single source provides the two input beams through optical fibers and optical fibers also guide the emerging beams to separate photodetectors for conversion to electrical signals which are subtracted to generate the output signal proportional to current. No provision is made, however, to accommodate for variations in the attenuation characteristics of the optical fibers which are very sensitive to movment and vibration, and for variations in photodetector sensitivity.